Greatwall (GW) is a new kinase that has an important function in the activation and the maintenance of cyclin B-Cdc2 activity. Although the mechanism by which it induces this effect is unknown, it has been suggested that GW could maintain cyclin B-Cdc2 activity by regulating its activation loop. Using Xenopus egg extracts, we show that GW depletion promotes mitotic exit, even in the presence of a high cyclin B-Cdc2 activity by inducing dephosphorylation of mitotic substrates. These results indicate that GW does not maintain the mitotic state by regulating the cyclin B-Cdc2 activation loop but by regulating a phosphatase. This phosphatase is PP2A; we show that (1) PP2A binds GW, (2) the inhibition or the specific depletion of this phosphatase from mitotic extracts rescues the phenotype induced by GW inactivation and (3) the PP2A-dependent dephosphorylation of cyclin B-Cdc2 substrates is increased in GW-depleted Xenopus egg extracts. These results suggest that mitotic entry and maintenance is not only mediated by the activation of cyclin B-Cdc2 but also by the regulation of PP2A by GW.